Star Spy: Rings of light

04:30, Oct 23 2012
Star Trails
Star Trails
Star Trails
Star Trails
Alpha Centauri
Alpha Centauri
Centaurus Crux Region
Centaurus Crux Region


Starlight. This is what you get through the simple process of aiming your camera at the South Celestial Pole (SCP) and keeping the shutter open, in this case for 30 minutes. 

The key here is that the camera doesn’t move to follow the stars, no tracking mount required, just a good tripod. A quick exposure of the area captures the objects. A long exposure captures their motion in the sky.

It is like taking a timed exposure of a busy city street at night. You have probably seen them, bright streaks of headlights on one side of the road, and red streaks of taillights on the other side. We actually have colours in the stars as well, but more subtle than car lights.

Earth turns on its axis and for us in the southern hemisphere its axis points south to the SCP. The result is that the sky above us appears to rotate clockwise around that point. 

The SCP isn’t a special place in the universe, in fact, isn’t actually a place at all. It is just a direction in the sky, where our axis happens to point at the moment, and that changes over time. 


Earth turns on its axis through a full 360 degrees in just under 24 hours.  That is approximately 15 degrees an hour. Fraser recommends at least a 30-minute exposure, which will capture 7.5 degrees of motion around the pole, as in the photo. 

A basic exposure is ISO 200 f/4 for 30mins, so almost any lens will do.  Wide angle is recommended.  As long as you are in a dark sky area, no artificial light pollution around, and no moon, you can expose for as long as you like.

The above exposure was ISO 320, f/2.8 taken with a Canon 1DS mkII and a 16-35mm lens (16mm). 

The background glow is still quite bright so an aperture of f/4 - 5.6 should remove a lot of this.

Another method is to take shorter exposures (all of the same length).  They can be very simply combined into one long one using computer freeware available at or

Make sure the gap between each exposure is as short as possible so you don’t have noticeable gaps in the finished product. 

You can do a lot of impressive work with just a few simple tools and procedures.


From now on, whenever you look at Alpha Centauri in the night sky, you can know that there is a planet in orbit there. 

The newly discovered planet, announced just last week, orbits Alpha Centauri B, one of the three stars in orbit around each other in the Alpha Centauri System. 

Of the over 1000 extrasolar planets that have been discovered, this one especially grabs and stirs the imagination. 

It is the closest extra-solar planet found, in orbit in the closest star system to the Sun.

It is currently just 4.3 light years from us, and actually is moving closer all the time. 

The planet’s star, Alpha Centauri B, is a sun-like star, and the little planet is also similar to Earth, in size and probably in composition. 

Indications are that it is made of very similar stuff to Earth – a rocky planet, not a gas giant like Jupiter or Saturn. 

However, the newly found planet occupies an unfortunate position with respect to its star. It is just six million kilometers from the surface of the B star, about 1 10th of Mercury’s average distance from the Sun.

It is hard to imagine how overwhelmingly large and bright the planet’s parent star must appear in its sky. 

Alpha Centauri Bb completes one orbit (its year) in under three days and six hours, and its surface temperature is a well-beyond-balmy 1200 degrees C.  Much of the planet is probably molten, a melted rocky planet. 

But not to worry, as, based on discoveries made to date, the statistical likelihood of finding other similar planets in the system is significant.  Hopefully some will be just comfortably warm. 

So watch this space and keep an eye on Alpha Centauri.